1. Field of the Invention
The present invention relates to a vibration prevention control circuit which is equipped in an imaging device.
2. Description of the Related Art
Recently, imaging devices such as a digital still camera and a digital video camera realize improved image quality by increasing a number of pixels of an imaging element provided in the imaging device. On the other hand, as another method of realizing improved image quality for the imaging device, it is desired for the imaging device to have a vibration correction function in order to prevent vibration of an image of an object caused by vibration of the hand holding the imaging device.
More specifically, a detection element such as a gyro sensor is provided in an imaging device, and optical components such as the lens and the imaging element are driven according to an angular velocity component caused by vibration of the imaging device, to prevent vibration of the object image. With this structure, even if the imaging device is vibrated, the component of the vibration is not reflected in the obtained image signal, and an image signal having no image vibration and a high image quality can be acquired.
FIG. 5 is a block diagram of a vibration prevention control circuit 100 of the related art which is used for realizing the vibration prevention function. The vibration prevention control circuit 100 is provided in an imaging device, and operates according to control of a main control circuit (not shown) provided in the imaging device. The vibration prevention control circuit 100 is connected to a position detection element 102, a lens driving element 104, and a vibration detection element 106.
The position detection element 102 detects a position of a lens which is used in the imaging device. A hall element may be used as the position detection element 102. The hall element produces an inductive current corresponding to an absolute position of the lens and outputs a voltage signal to the vibration prevention control circuit 100. A voice coil motor may be used as the lens driving element 104. The vibration prevention control circuit 100 controls a position of a movable coil of the voice coil motor, that is, a position of the lens with respect to an optical axis which forms a reference, by adjusting the value of the voltage to be applied to the lens driving element 104. The lens driving element 104 drives the lens within a plane which is perpendicular to the reference optical axis of the imaging device. The vibration detection element 106 detects vibration of the imaging device and outputs the result of the detection to the vibration prevention control circuit 100. A gyro sensor may be employed as the vibration detection element 106. The vibration detection element 106 generates an angular velocity signal corresponding to the vibration applied to the imaging device and outputs the angular velocity signal to the vibration prevention control circuit 100.
For each of the position detection element 102, the lens driving element 104, and the vibration detection element 106, it is desired for at least two elements to be provided. For example, a plurality of elements are provided corresponding to a horizontal component and a vertical component in a plane perpendicular to the optical axis of the imaging device, and the lens position detection, lens movement, and vibration detection of the imaging device are executed.
Next, the vibration prevention control circuit 100 will be described in detail. The vibration prevention control circuit 100 comprises a servo circuit 10, a lens driver 12, an analog-to-digital converter circuit (ADC) 14, a CPU 16, and a digital-to-analog converter circuit (DAC) 18.
The servo circuit 10 generates a signal for controlling the lens driving element 104 according to the voltage signal which is output from the position detection element 102. The servo circuit 10 comprises an analog filter circuit including an external resistor element, a capacitor, etc., and generates a signal which controls the lens driving element 104 such that the optical axis of the lens matches the center of the imaging element provided in the imaging device. The lens driver 12 generates a lens driving signal which drives the lens driving element 104 based on the signal which is output from the servo circuit 10.
The ADC 14 converts the analog angular velocity signal which is output from the vibration detection element 106 into a digital signal. The CPU 16 generates an angle signal which indicates an amount of movement of the imaging device based on the digital angular velocity signal. The CPU 16 is connected to a memory (not shown) and executes the generation process of the angle signal based on software stored in the memory. The DAC 18 converts the digital angle signal generated by the CPU 16 into an analog signal.
The servo circuit 10 generates a signal which controls the lens driving element 104 according to a signal in which the analog angle signal which is output from the DAC 18 and the voltage signal which is output from the position detection element 102 are added. In other words, in order to prevent vibration of an object image due to hand vibration, the position of the lens is changed based on the angle signal indicating the amount of movement of the imaging device, to inhibit vibration of the image of the object on the imaging element. With this structure, the vibration of the object image due to the vibration of the hand can be inhibited and an image signal of high image quality can be obtained.
In order to facilitate changing of an adjustment value of the filter provided in the vibration prevention control circuit, it is desired to replace the servo circuit, the lens driver, and the processor circuit of the vibration detection signal with logic circuits which can digitally process. In addition, because the vibration prevention control circuit is equipped in an imaging element such as a digital camera or the like or a lens module of the imaging element, the size must be minimized even when logic circuits are employed.
The angular velocity signal which is output from the vibration detection element 106 is integrated so that the angular velocity signal is converted into a signal indicating the angle (position) of the vibration, and the signal is used as a reference for comparison to a signal indicating the position of the optical element which is output from the position detection element 102, so that a driving signal which controls the position of the optical element is generated.
In this case, because the angular velocity signal can be considered as a superposition of sine waves (or cosine waves), when the angular velocity signal is integrated with respect to time, the phase is shifted by 90°. However, the vibration prevention control circuit includes a circuit which processes the angular velocity signal, and the phase of the angle signal obtained by the integration circuit would be deviated by these circuits. Because of this, a signal in which the phase is shifted by 90° cannot be obtained. There is a problem in that, because of this deviation, the angle (position) signal obtained based on the output signal from the vibration detection element 106 cannot be accurately compared to the position signal which is output from the position detection element 102.